Silica sols which are substantially free from metal impurities (Fe, Cr, Ni, Cu, etc.) have been proposed. Such high purity silica sol is suitable for use as an abrasive for electronic materials such as semiconductor silicon wafer because presence of such metal impurities in the silica sol results in the dispersion of the metal impurities into the interior of the wafer during polishing of the wafer and loss of wafer quality, and hence, in the poor performance of the semiconductor device formed by using such wafer.
Examples of the high purity silica sol that has been proposed include those disclosed in the Patent Literatures 1 to 3.
Patent Literature 1 discloses a method for producing a colloidal silica comprising the steps of contacting an aqueous solution of an alkali silicate with a cation-exchange resin to prepare an aqueous solution of activated silicic acid, contacting the aqueous solution of activated silicic acid with a chelating agent, allowing development of the colloid particles, and concentrating the silica by ultrafiltration simultaneously with the removal of the chelated metal impurities. Also disclosed is that the colloidal silica produced has a Cu content in relation to silica of up to 100 ppb or a Ni content in relation to silica of up to 1000 ppb.
Patent Literature 2 discloses a production method comprising the steps of preparing a solution of aqueous solution of an alkali metal silicate salt having a strong acid or a salt of a strong acid added thereto, treating the solution with an ion-exchange resin, adding another solution obtained by similar steps to the solution obtained by the ion exchange to prepare a silica sol, treating the resulting silica sol with an ion-exchange resin, and adding ammonia to the resulting silica sol. Also disclosed is that such production method is capable of producing a stable aqueous colloidal silica having an average particle diameter in the range of 10 to 30 mμ, a SiO2 concentration of 30 to 50% by weight, and content of polyvalent metal oxides other than the silica in relation to the silica of up to 300 ppm from an alkali metal silicate salt containing a polyvalent metal oxides as impurities.
Patent Literature 3 discloses a method for producing a high purity colloidal silica comprising the first step of subjecting water glass to cation exchange treatment to prepare an aqueous silica solution, the second step of adding an acid and hydrogen peroxide to the aqueous silica solution to conduct another cation exchange treatment, and the third step of mixing the aqueous silica solution with ammoniac alkali for preparation of the colloidal silica. Also disclosed is that such production method is capable of producing a high purity colloidal silica and a high purity synthetic quartz powder adapted for various applications such as an abrasive agent for a silicon wafer, binding of a ceramic fiber, an adhesion binder for a phosphor in the CRT production, and a gelation agent of an electrolyte in a battery.